1. Field of the Invention
The present invention relates to improved methods of fracturing subterranean zones and improved fracturing fluids.
2. Description of the Prior Art
Viscous treating fluids are used in a variety of operations and treatments in hydrocarbon producing wells. One such treatment for stimulating hydrocarbon production from a subterranean zone penetrated by a well bore is hydraulic fracturing. In a hydraulic fracturing treatment, a viscous treating fluid, referred to in the art as a fracturing fluid, is pumped through the well bore into the subterranean zone to be stimulated at a rate and pressure such that fractures are formed and extended in the subterranean zone. At least a portion of the fracturing fluid has proppant material suspended therein which is carried into and deposited in the formed fractures when the viscous fracturing fluid is broken and recovered. The proppant material functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore. The breaking of the fracturing fluid, i.e., the reduction in the viscosity of the fracturing fluid whereby it can be recovered is accomplished by adding a delayed breaker to the fracturing fluid prior to pumping it into the subterranean zone. The delayed breaker effects a controlled reduction in the viscosity of the fracturing fluid so that the proppant material therein is deposited in the fractures and the fracturing fluid is recovered.
A variety of aqueous fracturing fluids which utilize natural or synthetic viscosity increasing polymers have been developed and used heretofore. Because of their cost effectiveness, natural polymers formed of guar gum and its derivatives have been the polymers of choice for forming viscous aqueous fracturing fluids. However, in high temperature applications, aqueous fracturing fluids containing guar gum and its derivatives degrade and lose viscosity. As a result, a variety of improvements designed to prevent or reduce the degradation and loss of viscosity of aqueous guar fracturing fluids have heretofore been made. One of the improvements has been to prevent degradation through hydrolysis of the acetal linkages in the backbone of guar polymers by raising the pH of the fracturing fluid. The higher pH reduces the susceptibility of the acetal linkages to hydrolysis. Additionally, gel stabilizers such as methanol and sodium thiosulfate have been added to the aqueous guar fracturing fluids to minimize oxidative degradation. Other techniques include adding chelating agents and/or various free-radical scavengers to the guar fracturing fluids and using precross-linked guar powder to delay hydration and extend the life of the fracturing fluid at elevated temperatures. While these techniques have reduced the degradation of aqueous guar fracturing fluids, they have not eliminated it.
Various viscosity increasing synthetic polymers have been developed for use in aqueous fracturing fluids that can be cross-linked to achieve high viscosity and subsequently broken. While such synthetic polymers have achieved some success, there are continuing needs for improved viscous fracturing fluids which achieve improved performance in fracturing treatments generally and do not suffer from instability at high temperatures.